The present invention generally pertains to injection molding of plastic products and is particularly directed to balanced multi-cavity injection molding of plurality of like products.
One type of product for which it is a difficult to provide balanced filling of the mold cavities when injection molding a plurality of like products with a multi-cavity mold is a ridged-wall product including a plurality of longitudinal folds of alternating ridges and furrows, with each fold including a first side strip laterally extending from the top of a said ridge to the bottom of a said furrow, and a second side strip laterally extending from the bottom of the said furrow to the top of a said ridge next to the said ridge; wherein the first side strip has a different lateral orientation than the second side strip; and wherein the first side strip has a greater wall thickness than the second side strip. Such a product is described in U.S. Pat. No. 5,267,685 to Jens Ole Sorensen.
For efficient manufacture, it is preferred to injection mold such products simultaneously with a multi-cavity mold. A problem incident to multi-cavity injection molding of such products is that all of the cavities of the mold in which the products are formed are not always completely filled simultaneously with the injected plastic material. Referring to FIG. 1, which illustrates a plurality of injection molded hollow ridged-wall plastic products 10, 10a, 10b remaining in one mold part 12 after the mold parts 12, 14 defining the respective cavities in which the products are formed have been separated, it is seen that failure to completely fill all of the mold cavities simultaneously results in some of the cavities being incompletely filled, such that a product 10a therein is not completely formed, and other cavities being overfilled, such that a flashing of excess plastic 16 is attached to a product 10b formed therein. The gates through which the plastic material is injected into the mold cavities are located at the center of the end of the respective cavities at which closed ends 18 of the hollow plastic products 10, 10a, 10b are formed.